1. Field of the Invention
The present invention relates to a powdery polyurethane lacquer useful for the coating of substrates by electrostatic application.
2. Description of the Prior Art
It is already known to use powdery lacquers made of hydroxy group-containing polyesters, polyacrylates and epoxide resins with polyisocyanates with blocked isocyanate groups in the lacquer sector, for the coating of a substrate by electrostatic application by spraying or drizzling, followed by hardening.
As a blocking agent for the isocyanate groups it has been found, after some experiences with phenol (which has been unsatisfactory with regard to odor problems and bubble formation) to use especially .epsilon.-caprolactam (DT-OS 19 57 483).
As polyisocyanate, it has been known to use, because of a series of advantages, especially 3-isocyanatomethyl-3,5,5-trimethylcyclohexylioscyanate, named as IPDI hereinafter (DT-AS 21 05 777).
Polyurethane-powdery lacquers on the basis of .epsilon.-caprolactam-blocked IPDI are characterized, with respect to polyurethane powdery lacquers made on the basis of other polyisocyanates by especially good weather stability, flow properties and thermostability. They have the disadvantage however-compared for example with epoxide resin powders- that the hardening conditions, that is the hardening temperature and hardening time, are relatively high.
Many attempts to decrease these disadvantages as for example by addition of catalysts which can reduce the hardening times, have been made. However, because of technological disadvantages, it has been impossible to carry out these proposals.
Compared with the epoxide resin powder lacquers, and also with others, the weight relation between resin (polyester) to hardener is, in polyurethanes, clearly on the side of the hardener. This fact burdens not only the commercialization of powder lacquers based on this resin group, but also requires a special standardization on the part of the production plants due to this circumstance.
These disadvantages can be dealt with by two means. In the case where it is necessary for this resin group to have a stoichiometric relation of resin/hardener for the standardization of the optimum group of properties, it is possible theoretically to push the relation between resin/hardener in favor of the resin in such a way that one tries to increase the content of the cross-linked isocyanate groups and/or to reduce the content of the OH-groups on the resin side. If one follows the first option, then one finds out that, based on the presently used isocyanate-urethane hardeners, an increase in the NCO-concentration in the hardener influences in a strong negative way the strong stability of the finished powder lacquer. The reduction in the OH-count of the polyol component of the polyester leads usually to a decrease in the functionality of the polyol component. Because of it, the chemical stability of such a lacquer is very weak, and it has to be made up desirably by increasing the functionality of the hardener.